Patent GB-830529-A (GB)
Title: Improvements in or relating to electric control arrangements
IPC: F23N5/08
Filed: 1957-06-05
Abstract: 830,529. Controlling burners. MEIKLE, O., FIANDER. L. P., and ANDREWS, D. J. G. May 30, 1958 [June 5, 1957], No. 17884/57. Class 75(1) [Also in Groups XL(b) and XL(,c)] An electric control arrangement for an oil or gas burning furnace comprises a relay device, two capacity resistance timing circuits connected to a control electrode A3 of a thermionic valve A, means whereby a first stage of operation of the relay device controlling the starting up of the furnace can be effected only after expiration of the time delay of the first timing circuit and means whereby a second stage of operation of the relay device controlling safeguards in the event of faulty starting up of the furnace can be effected only after expiration of the time delay of the second timing circuit. The first timing circuit comprises the condenser Gland the resistance G5 and the delay is initiated by the relay device reverting to its de-energized condition at the end of a burner operating period. The second timing circuit comprises the same condenser Gl and the fixed resistances G2, G4 and the variable resistance G3 and provides a time interval for the operation of a flame detector. The relay device comprises a first relay D energized by the anode current of the valve A, a second relay E controlled by the first relay D and a third relay F controlled by the second relay E and adapted to energize'an electric motor J2 supplying oil, fuel and air to the burner (not shown) or an electromagnetic valve supplying gas to the burner. The burner is ignited by an electric spark controlled by an ignition transformer J. The burner flame is detected by a thermostat H exposed to the heat of the flame or by a device responsive to the electrical conductivity of the flame or by a photo-electric cell exposed to the light of the flame. The burner is controlled by condition responsive devices C3 such as furnace, flue, hot water and room thermostats and steam pressure responsive devices, connected between the live terminal C1 of an A.C. source and a bus bar C2 to which the input terminal of the flame detecting thermostat H is connected. The relay contacts are shown in the de-energized condition of the relays. In operation on closure of the condition responsive devices C3 the anode of the valve is connected to the live terminal C1 through the cold contact H1 of the flame detector H and provided the time delay of the first timing circuit has expired, the valve conducts and energizes the first relay D. The contact D1 energizes the second relay E, the contact D2 prepares a point in the energizing circuit of the third relay F and the contact D3 prepares a point in the energizing circuit of the ignition transformer J. The second relay E therefore operates. At its first contact E1 it breaks a shunt circuit across the condenser G1, at its second contact E2 it completes the circuit of the third relay F, and at its third contact E3 it completes its own holding circuit through the cold contact H1 of the flame detector H. The third relay F therefore operates. At its first contact F1 it breaks another point in the shunt circuit across the condenser G1; at its second contact F2 it breaks the connection between the condenser Gland the neutral terminal C of the source thus initiating the time delay of the second timing circuit G1, G2, G3, G4; at its third contact F3 it closes a point in its own holding circuit which however at this stage is broken both at the second contact E2 of the second relay E and at the hot contact H2 of the flame detector H; at its fourth contact F4, which is a make before break contact, it transfers the energizing circuit of the first relay D from the cold contact H2 of the flame detector to the live terminal C1 of the source; and at its fifth contact F5 it completes the energizing circuit to the burner motor J2 and to the ignition transformer J. Fuel and air are thus supplied to the burner and an ignition spark generated. If the burner ignites within the time delay of the second timing circuit the flame detector opens its cold contact H1 and closes its hot contact H2, thus breaking the holding circuit of the second relay E, which however remains energized through contact D1, and closing a further point in the holding circuit of the third relay F which however remains broken at the contact E2. The second timing circuit causes a build up of negative charge on the grid of the valve and the anode current eventually falls below the value necessary to hold the first relay D energized. The first contact D1 then de-energizes the second relay E, the second contact D2 begins to open slowly and the third contact D3 deenergizes the ignition transformer J. On de-energization the second relay E at its second contact E2 completes the holding circuit of the third relay F through the hot contact H2 of the flame detector, this circuit being completed before the initial energizing circuit of the third relay F is broken since the second contact D2 is slow to open and the second contact E2 is of the make before break type. The third relay F therefore remains energized and the other two relays D, E de-energized, in the normal operating conditions of the furnace. The burner continues to operate until a condition responsive device C3 deenergizes the third relay F. The contacts F1 and F2, then complete the first timing circuit G1, G5 and the negative charge on the grid leaks away so that after the time delay the valve is again ready to pro- 'vide an anode current sufficient to operate the first relay D. When the condition responsive devices C3 again close, the starting cycle is repeated. If the burner fails to ignite within the time period of the second timing circuit the first relay D will be de-energized before the flame detector H has moved from its cold contact H1 to its hot contact H2. Consequently the second relay E remains energized through the cold contact H1 and its own contact E3, and the third relay F is de-energized at the second contact D2. The shunt circuit across the condenser Gl remains broken at contact El so that the first timing circuit remains out of operation and the valve cannot pass current. This is a lock out condition and is indicated by energization of an alarm device K. The second relay E and the alarm device K are de-energized by a manual switch K1 to bring the control arrangement to the starting condition. In order to guard against failure of the first relay D to revert to its normal condition when the anode current falls below the holding value of the relay, a time-lag contactor J1 is connected in parallel with the ignition transformer J and is adapted to break the live terminal C1 after a period of energization appreciably longer than the time delay of the second timing circuit. The control circuit of Fig. 2 (not shown) uses different relays from those of Fig. 1 but the operation of the circuit is similar to that of Fig. 1. The control arrangement of Fig. 3 (not shown) is identical with that of Fig. 2 except that Fig. 3 includes a photo-electric cell directly energizing a two-way switch corresponding to. the contacts H1, H2 of the flame detector H. The photo-electric cell is energized by a rectifier fed from an additional winding on the step down transformer heating the filament of the valve. The fuel oil may be heated by an electrical heater, Pl, Fig. 4 (not shown) controlled by a thermostat P to keep the fuel at a predetermined temperature. A. further themostat P5 is connected in series with the condition responsive devices C3 and is normally closed but opens to deenergize the control arrangement in the event of the temperature of the fuel falling below a temperature at which the thermostat P controlling the electrical heater operates. In order to prevent cold fuel oil trapped between the heater and the burner being fed to the burner at starting a recirculation pipe extends from the burner nozzle to the supply side of the fuel heater and is provided with a solenoid operated valve Q which when open prevents a pressure operated valve in the burner nozzle from opening and supplying fuel to the furnace. The solenoid operated valve Q is connected in parallel with the burner motor J2 and is controlled by a relay S connected in the anode circuit of a second triode valve. The control grid of the second triode is connected to a third timing circuit comprising condenser T1 and a high resistance T2 and connected to an additional change-over contact F6 on the third relay F. When the latter is de-energized the third timing circuit is connected to the live terminal of the source so that the valve supplies anode current sufficient to open the contacts of the relay S and de-energize the solenoid valve Q. When the third relay F is energized to start the burner the additional contact F6 connects the third timing circuit to the neutral terminal of the source to allow a negative voltage on the grid to discharge' through the timing circuit. During this discharge time the anode current is insufficient to open the contacts of the relay S so that the solenoid valve Q is energized and the cold oil circulates to the heater. The control arrangement of Fig. 5 (not shown) is similar to that of Fig. 3 except that the photo-electric cell does not operate a two-way switch directly but instead is connected into the grid circuit of a further triode valve the anode current of which operates the two-way switch. Specification 785,033 is referred to.